1. Field of the Invention
This invention relates to polymeric adhesive compositions and their use in radiation sensitive elements, and more particularly in photographic materials such as diffusion transfer film units. In particular, this invention relates to heat-sealable non-pressure sensitive adhesive compositions useful at low temperatures comprising mixtures of vinylbenzenediolefin block copolymers and copolymers of alkenes and vinyl acetate.
2. Description of the Prior Art
It is well known that certain copolymers or copolymer blends are useful as adhesives. For instance, U.S. Pat. Nos. 2,485,248 of Watson et al. and 3,232,895 of Klein et al. relate to low temperature, pressure-sensitive adhesive compositions comprising a copolymer of ethylene and vinyl acetate. U.S. Pat. No. 3,378,424 of Sawyer discloses the use of adhesive compositions containing copolymers of ethylene and vinyl acetate in laminates. Hot melt coating agents and adhesives which comprise ethylene-vinyl acetate copolymers and resins prepared by copolymerizing vinyl aromatic hydrocarbons containing isopropenyltoluene and a by-product fraction obtained in refining or cracking petroleum are described in U.S. Pat. No. 3,875,095 of Yamada et al.
Copolymers of vinyl aromatics, such as styrene, and conjugated diolefins have also been used as adhesives as disclosed in U.S. Pat. Nos. 2,601,318 of Navikas; 2,751,322 of Bost; 3,242,038 of Dallas et al.; 3,459,700 of Richards; 3,645,817 of Walker et al.; and 3,850,858 of Park.
U.S. Pat. No. 3,630,980 of Russell discloses pressure-sensitive adhesives having good quick tack at room temperature. These adhesives comprise (a) an ethylene-vinyl acetate copolymer; (b) a block copolymer of styrene and a conjugated diolefin; and (c) a third resin, such as a coumarone-indene resin, polystyrene resin, etc, which provides instant tackiness and allows the adhesive blend to be made without the use of solvents. No photographic uses are disclosed for these adhesives.
In U.S. Pat. No. 3,644,252 of Shenfield et al. are disclosed hot-melt, pressure sensitive adhesive compositions useful at elevated temperatures (above 120.degree. C.) comprising random copolymers of styrene and olefins and copolymers of ethylene and vinyl acetate. No photographic uses are taught or suggested for these adhesives.
Compositions for making rigid, extruded articles are disclosed in U.S. Pat. No. 3,894,117 of Agouri et al. These compositions comprise an ethylene-vinyl acetate copolymer and a block copolymer of styrene and an olefin. No mixtures containing block copolymers of styrene and diolefins and no adhesive or photographic uses are taught or suggested.
Radial block thermoplastic rubbers can be used in a variety of adhesive applications, such as pressure sensitive adhesives, contact cements and hot melts as indicated by J. R. Haws in Organic Coatings and Plastic Preprints, 34(1), pages 114 through 121 (1974).
Various formats for photographic film units are described in U.S. Pat. Nos. 2,352,014 of Rott; 2,543,181 of Land; 2,983,606 of Rogers; 3,020,155 of Yackel et al.; 3,227,550 and 3,227,552 of Whitmore et al.; 3,415,644; 3,415,645, and 3,415,646, all of Land; and 3,635,707 of Cole; Canadian Pat. Nos. 674,082 of Whitmore, and 928,559 of Cole; U.S. Pat. Nos. 3,362,819; 3,362,821, and 3,647,437, all of Land; and 3,756,815 of Schlein et al.; Belgian Pat. Nos. 757,959 and 757,960; British Pat. Nos. 904,364 and 840,731; and copending U.S. application Ser. No. 676,945 of Hannie, and Ser. No. 676,947 of Hannie et al., both filed Apr. 14, 1976 and now U.S. Pat. Nos. 4,056,394 and 4,061,496 respectively. None of the prior art teaches or suggests the adhesive compositions disclosed herein or their use in photographic materials.
In these formats, the image-receiving layer containing the photographic image for viewing can be separated from the photographic layers after processing or, in some embodiments, it can remain permanently attached and integral with the image-generating and ancillary layers present in the structure when a transparent support is employed on the viewing side of the assemblage. The image is formed by color-providing substances released from the image-generating units, diffusing through the layers of the structure to the dye image-receiving layer. After exposure of the assemblage, an alkaline processing composition permeates the various layers to initiate development of the exposed photosensitive silver halide emulsion layers. The emulsion layers are developed in proportion to the extent of the respective exposures, and the color-providing materials which are formed or released in the respective image-generating layers begin to diffuse throughout the structure. At least a portion of the imagewise distributed or released color-providing substances migrate to the dye image-receiving layer to form an image of the original subject.
Typically, adhesives are used in photographic film units to bond supports, subbed or unsubbed, to cover sheets or other layers around the edges of the units. Adhesives can also be used to bond subbing layers to supports or cover sheets, photosensitive layers to image-receiving layers, and the like.
It has been desirable to provide high strength adhesives for bonding barrier timing layers to spacer rail substrates which generally separate the photosensitive element of the film unit from a barrier timing layer-containing cover sheet. However, due to the many prerequisites for acceptable bond strengths at low temperatures of barrier timing layers to spacer rail substrates, none of the prior art adhesives are useful in these photographic materials. Prior art adhesives sometimes have desired bond strengths at temperatures above 25.degree. C. and below 70.degree. C. but not below 25.degree. C., and especially not at 5.degree. C. or lower. The pressure sensitive (instant tackiness) adhesives are not useful in these image transfer units as they are not conducive to heat or dielectric sealing, long-term storage or transport on sealing machines. Further, prior art adhesives do not adequately adhere to some of the materials prominently used as barrier timing layers or spacer rail substrates.
It is evident, then, that there is a need in the photographic arts for heat-sealable, non-pressure sensitive adhesives which exhibit high bond strengths at temperatures below about 75.degree. C., and especially below 25.degree. C.